If you are using the Google calendar sync feature, each user can share their calendars through Google calendar settings.

Open the Google calendar page.

Select the gear icon and choose Settings from the dropdown menu.

Select the Calendars link in the upper right of the next page. All your calendars will display in a list.

On the line for the Insightly calendar you want to share, click on the Shared: Edit settings link on that line. (These will only appear if you have elected to sync tasks and milestones. Events are synced to your main calendar.)

Several sharing options will appear on the page to share by either organization or individual email address.